In machine automation, there is a growing trend of transition from centralized systems towards decentralized systems, in which intelligent functions, coordination between different components comprised in a system, diagnosing and control are increasing. Decentralized systems are network systems, in which the nodal units and the control units controlling them are connected to each other via signaling channels. The signaling channels used consist of wired cables. Decentralized systems are often designed with an aim to make it easy to integrate the nodal units or nodes with the devices or components to be monitored, thus allowing easy, reliable, cost-effective and, if desirable, real-time monitoring and control of the devices and components.
However, the applicability of decentralized systems has been limited by a lack of suitable data transfer solutions. In decentralized systems, each nodal unit usually has an address by means of which the control unit identifies the nodal unit to read or control its state. To allow the state of the nodal unit to be read or controlled, it is also necessary to have a clock signal for the timing of the signaling. The use of an address and a clock signal makes especially nodal units complicated digital state machines that require space. To enable data to be transferred between the nodal unit and the control unit in serial mode, it is further necessary to have a data transfer protocol and protocol circuits, which require regulated operating voltages. The supply of operating voltage together with the regulating circuits makes the structure of the nodal units still more complicated. The software for the protocol and other functions again requires plenty of development work and is usually intrinsic to each commercial system, which further complicates the use of network systems. Because of these problems, network systems are expensive to acquire, operate and maintain.